Fill Tool ignores Grease Pencil Lines?
This Sounds like you have a gap and hence filling the screen. This depends allot on your line and Fill thickness/leakage thickness. The reason it's filling from a distance is the fill resolution/ thickness/ leakage is relative so a tiny gap when zoomed in disappears when you zoom out.
The gap might be tiny but heres some options for you;

turn up the fill resolution (in advanced drop down). I keep mine at 3 or the fill shape sucks.
increase your fill tool leakage size
Join your strokes to remove gaps. Select the strokes and use ctrl J.
If you only have 1 stroke use F to auto close it.

One of those should sort your issue.
Good luck with it.